Speaking after meeting the players at his London office, Hasan said they had insisted they were innocent but were pulling out of the tour because of the "mental torture" of the scandal.

"The three players have said that they are extremely disturbed by what has happened in the past week, especially in regard of their alleged involvement in the crime," he told reporters.

"They mentioned that they are entirely innocent in the whole episode and shall defend their innocence as such.

"They maintain that on account of the mental torture which has deeply affected them, they are not in the right frame of mind to play the remaining matches.

"Therefore, they have requested that the Pakistan Cricket Board not consider them for the remaining matches."

Asked later if he believed the trio were innocent, Hasan replied: "Yes, I believe in their innocence."

Butt, Aamer and Asif were all named in a News of the World report which alleged they were involved in a "spot-fixing" scam by bowling deliberate no-balls in last week's Test match with England in exchange for cash.

However Hasan later questioned the authenticity of video footage shot by the newspaper, saying the players may have been framed.

"The video wasn't timed or dated. It could have been filmed before or after the match, or at a different time," he told the BBC.

Asked specifically whether he believed the players may have been set up, he replied: "Yes, I would say that. Yes."

The revelations have shocked the cricket world and led to calls from figures within the game that the players involved should be banned for life.

The trio were quizzed by police during the Lord's Test and had their mobile phones confiscated as detectives investigated the allegations.

Sky News reported that police were planning to question them again on Friday, although Scotland Yard would not confirm this.

London-based businessman Mazhar Majeed was arrested on Sunday but released without charge on police bail.

Meanwhile Australian Test legend Shane Warne added his voice to the chorus of condemnation, saying authorities should choose the ultimate punishment if the three players were found guilty.

"If it is true and they have been found (guilty of) match-fixing and throwing games and spot-betting with the no-balls and stuff, if that's the case they should be thrown out," Warne told reporters. "It's as simple as that."

Former England cricket coach Duncan Fletcher also adopted a hard line, writing in The Guardian newspaper that life bans were the only way to send a clear message to players who compromised the integrity of the sport.

"We must be ruthless and put the fear of God into people. Even the smallest transgression must mean that a career is over," he said.

Meanwhile Pakistan's tour of England continued on Thursday with the side registering a victory by eight runs over county side Somerset.

The match passed off without incident and a collection amongst the crowd of 4,000, who acknowledged the tourists' play with polite applause, raised 2,580 pounds (3,971 dollars) for the Pakistan flood-relief fund.

However, the team cancelled a scheduled training session in Cardiff on Friday, ahead of the first Twenty20 international on Sunday, according to the England and Wales Cricket Board.